I can understand standing his ground, for sure, but there's a difference between standing your ground and allowing people to **** all over you, and it looks like he did this more than give any kind of powerful message to the bullies.
I'd have at least asserted myself...but then, some peoples self-esteem ain't too high. Time for him to work on this I'd say. I would probably also not stand around and just watch someone get treated like this. Power in numbers means that the more people around him on his side, the less the bullies will bully.
It's always easy to think about it when you're not the one experiencing it though.